The invention relates to a hydraulic chain tensioner for use with an associated chain or belt that drives an engine camshaft, balance shaft drive or the like. In particular, the hydraulic tensioner of the present invention is directed to an improvement of the construction of a hydraulic tensioner including a device to control the venting of air and fluids in cooperation with a device to provide pressure relief.
In general, a hydraulic tensioner includes a housing, a plunger inserted into a bore formed in the housing, and a spring that biases the plunger in the projecting or protruding direction. In the housing, a chamber is formed by assembling a hollow plunger into the bore of the housing. Oil is supplied to the chamber from an external oil pressure source. The pressurized oil acts on the plunger to cause the plunger to protrude from the housing bore and provide tension to an associated chain.
In a hydraulic tensioner, when air mixes with oil in the chamber, the plunger can be forced inwardly toward the housing due to compression of the mixed air and oil. The air/oil mixture compresses when the tension of the chain or belt increases and, as a result, the chain or belt tension can vary due to the reduced effectiveness of the tensioner.
A hydraulic tensioner, as disclosed in Japanese Laid-Open Patent No. 7-158703, discloses one proposed solution of such a problem. This hydraulic tensioner has a disc as a vent device inside the plunger. A spiral groove is formed on the disc surface. The starting end of the groove is located on the outer peripheral side of the disc surface and the terminal end is located at the center of the disc surface.
FIG. 6 is an enlargement showing the plunger tip part or upper end of the above hydraulic tensioner disclosed in the above patent publication. In the FIG., open hole 100a is formed at the tip of plunger 100 and disc 110 is inserted into hole 100b formed inside plunger 100. Spiral groove 110a is formed on one main surface (top surface) of disc 110 and groove 110b that extends in the direction of the center axis and connects the starting end to the terminal end of groove 110a and is formed on a part of the outer peripheral face of disc 110.
In such a hydraulic tensioner, the air mixed in the chamber passes through spiral groove 110a from groove 110b on the outer periphery of disc 110 and open hole 100a of plunger 100 and leaks out to the outside air together with hydraulic oil. At such times, excessive leaking of hydraulic oil in the chamber to the outside of the tensioner is suppressed because of the spiral shape of groove 110a on the main face of disc 110.
On the other hand, as shown in Laid-Open Patent Heisei 9-119490, a hydraulic tensioner with a vent that has a pressure relief valve, is also proposed. This pressure relief valve permits flow of fluid in the chamber, when the fluid pressure exceeds a set maximum value during operation, to reduce the fluid pressure in the chamber.
In the hydraulic tensioner of the above patent publication, an air vent disc is provided at the plunger tip and a pressure relief valve is provided on the bottom wall of the housing. When such hydraulic tensioner is assembled, the assembly of the air vent into the plunger and assembly of the pressure relief valve into the housing are separate processes and labor and cost increase.
The present invention eliminates this conventional problem and its objective is to offer a hydraulic tensioner with a reduced cost of assembly and smoothes the flow of air from the vent device.
It is a first object of the invention to provide a hydraulic tensioner including a housing with a bore that opens on one end and a hollow plunger that is slidably received by the bore. The plunger, or piston has a tip or upper end with an open hole or aperture and a spring that pushes biases the plunger in a protruding direction. A pressure relief valve assembly that permits the flow of fluid from the chamber under a present specific pressure condition, and a vent device is located inside of and at the tip of the plunger and is provided integrally with the pressure relief valve assembly. The vent device has a disc shape composed of first and second main surfaces that are opposite each other and an outer periphery surface located between those main surfaces. The outer diameter of the vent device is made slightly smaller than the inner diameter of the plunger. The vent has a through-hole that opens at the center of the first and second main surfaces, and a chamfer or radius is formed on the peripheral edge of the first main surface of the vent device. The starting end of the vent passage is located at the chamfer or radius and the terminal end of the vent passage is located at near center of the first main surface.
In the assembly of the hydraulic tensioner the second main surface of the vent device is placed opposite the pressure relief valve assembly and these components are inserted into the plunger, in which the vent device and the pressure relief valve assembly are integrally assembled. The first main surface, with a vent passage of the vent device, is opposite the plunger tip.
In this case, a vent device and pressure relief valve assembly, integrally assembled, are assembled into the plunger, so that one process can assemble these two components (i.e., at one station of the assembly machine) and thereby, labor and assembly costs can be reduced.
In operation air mixed into the chamber passes through the gap between the outer periphery of the vent device, the inner periphery of the plunger, and the chamfer or radius on the peripheral edge of the first main surface of the vent device, and flows into the starting end of the vent passage. Then the mixed-in air flows to the terminal end part vent passage and is discharged to the outside air from an open hole at the tip of the plunger.
In this case, mixed-in air flows toward the first main surface of the vent device by passing the optional position on the outer periphery of the vent device because a gap is formed between the outer periphery of the vent device and the inner wall or inner periphery of the plunger. The mixed-in air flows smoothly to the starting end of the vent passage past the chamfer or radius because the starting end of the vent passage is located at the chamfer on the peripheral edge of the first main surface of the vent device or the curved radius, and it flows outside the tensioner from the terminal end past the vent passages. In this manner, flow of the mixed-in air from the vent device is made smooth.
When pressure in the chamber becomes high, the fluid in the chamber flows out of the tensioner through the through-hole of the vent device and an open hole at the tip of the plunger, because of the pressure relief valve, thereby, abnormally high pressure in the chamber is prevented.
It is a second object of the invention to provide the hydraulic tensioner with an axial groove or notch that connects to the chamfer or radius formed on the outer periphery surface of the vent device. An axial groove or notch, that connects to the chamfer or radius, is formed on the outer periphery of the vent device. Therefore, the mixed-in air in the chamber moves to the first main surface of the vent device through the groove or notch during operation. Next, it flows to the starting end of the vent passage via the chamfer on the peripheral part or radius and is discharged out of the tensioner via the vent passage.
It is a third object of the invention to provide a circuitous vent passage that runs from the starting end to the terminal end so that the fluid flow from the chamber is restricted and the volume of leakage is suppressed.
It is a fourth object of the invention to provide a spiral vent passage.
It is a fifth object of the invention to provide the starting end of the vent passage tapered down toward its tip. The starting end of the vent passage is tapered down toward the tip, so that the flow of a large volume of fluid into the starting end of the vent passage is suppressed and the leakage flow can be limited.
It is a sixth object of the invention to provide a vent device urged toward the plunger tip by the spring. The vent device is urged toward the plunger tip by the spring so that, during assembly, the first main surface of the vent device contacts the inner wall at the plunger tip.
It is a seventh object of the invention to provide a further passage, for connection of the chamber to the external pressurized fluid source, the passage located inside the housing, so that the fluid from the external pressurized fluid source is supplied to the chamber via the passage.
It is an eighth object of the invention to provide a check valve that permits fluid to flow to the chamber and blocks the reversed flow of the fluid between the chamber and the passage so that the fluid flow to the chamber from the pressurized fluid source is permitted while its reverse flow is blocked.
For a more complete understanding of the invention, one should refer to the embodiments illustrated in greater detail in the drawings and described below by way of examples of the invention.